Currently the digitalization of radio communication is in full scale progress. In the United States, Japan and Europe, digital cellular systems according to the TDMA scheme are in practical use based on regional requirements, and each system has its own standard. The standard includes specifications for the minimum performance of a transmitter used in a particular system or a method of evaluation therefor. A standard for the digital cellular system according to the CDMA scheme is being formulated in the TR 45.5 Subcommittee of TIA/EIA, sponsored by QUALCOMM Company, and the evaluation of the performance of the transmitters and the receivers are defined by the standards IS-98 and IS-97. The present invention is directed to a system for measuring parameters which are required in performing "waveform quality measurement" defined in the standards IS-98 and IS-97 as well as the waveform quality measurement of digital quadrature modulation signals such as PSK, FSK: QAM or the like. In particular, the invention relates to a system for measuring parameters such as carrier frequency error, carrier phase, clock (symbol) phase or timing by processing digital data that is obtained by down-converting an RF (radio frequency) transmitted signal to be measured using a spectrum analyzer, sampling it at an appropriate sampling rate, and quantizing samples with an A/D converter having a suitable number of bits.
An apparatus for measuring modulation accuracy for a digital cellular system according to the TDMA scheme such as NADC is already developed, and is disclosed, for example, in U.S. Pat. No. 5,187,719, issued Feb. 16, 1993, in particular, in FIG. 15 thereof. A general arrangement is shown in FIG. 1 where an RF signal to be measured from an input terminal t.sub.1 is converted into an IF measuring signal having a frequency determined by a frequency converter 2 which utilizes a signal from a local oscillator 1, and is then passed through an analog low pass filter 3 in order to eliminate frequency components outside a frequency band of interest. An output from the filter is sampled and quantized in an A/D converter 4, and the resulting digital data is stored in a buffer memory 5. The IF signal stored in the buffer memory 5 is processed by a digital signal processor 6 to provide a final measure.
Referring to FIG. 2, the digital signal processor 6 includes a baseband signal converter 7 which converts the IF measuring signal from the memory 5 into a baseband measuring signal having a spectral content around zero frequency, which signal is then translated into a signal form which is appropriate to effecting a calculation of a desired item to be measured, by a baseband signal correction unit 8, which also generates a reference signal required in order to calculate a desired item to be measured. Finally, the desired item to be measured is processed in a desired item calculation unit 9 in accordance with a signal processing algorithm as described in the cited U.S. Patent, for example.
Referring to FIG. 3, which shows the detail of the signal processing, the IF measuring signal from a terminal 10 is branched, before it is fed to the baseband signal converter 7, into a clock phase estimation unit 71 where a clock (symbol synchronization) phase is estimated. On the basis of this estimated phase, the input IF signal is resampled in a resampler 72 using an interpolation technique. The resampled output is converted into a baseband measuring signal by the baseband signal converter 7. A portion of FIG. 3 which follows the baseband signal converter until the signal is input to a desired item calculation unit 79 corresponds to the signal correction unit 8 shown in FIG. 2.
The signal correction begins with a demodulation of transmitted data from the input baseband measuring signal which takes place in a data detector 73. At this end, a clock phase or symbol synchronizing phase is supplied from the clock phase estimation unit 71. The detection of transmitted data which takes place here corresponds to the so-called delayed detection, allowing such detection in the presence of a frequency error or a phase error since output signals from the converter 7 contain frequency and/or phase error which is occurring prior to the converter 7. Demodulated data which is output from the data detector 73 is used to specify a time position in a TDMA burst in a time reference extractor 74. Specifically, a predetermined data pattern (or sync word) is delivered at a specified time position within a burst, and accordingly, a time position can be specified by detecting the sync word. Demodulated data is then fed to a reference signal generator 76 which generates a reference signal. On the other hand, a correction of the baseband measuring signal is made in a signal correction unit 75, which utilizes the baseband measuring signal and the reference signal from the generator 76 to perform the following operations:
1. parameters such as frequency error, phase error or the like (hereafter collectively referred to as transmission parameters) contained in the baseband measuring signal are estimated; PA1 2. these estimated transmission parameters are used in forming a coherent complex sinusoidal wave, which is multiplied by the baseband measuring signal; and PA1 3. IQ origin offset is estimated and subtracted from the complex sinusoidal signal formed.
A correction of the baseband measuring signal takes place in a manner mentioned above. The corrected measuring signal is filtered by a root Nyquist filter 78 to provide a signal waveform from which a inter-symbol-interference is removed. Subsequently, the signal is input to the signal correction unit 75 where the processing operations mentioned under the sub-paragraphs 1 to 3 are repeated. Thus, a correction of the signal by the signal correction unit 75 is repeated several times, and is completed when a variation is reduced below a predetermined threshold value. Finally, the corrected measuring signal is fed to a desired item calculation unit 79. However, in the prior art, it happened that the repetition fails to converge. What is mentioned is an example of measuring a modulation accuracy of the prior art, as exemplified by the cited patent. This algorithm premises a modulation scheme of .pi./4 DQPSK, and is inapplicable to OQPSK (offset QPSK) signal. To illustrate, the measuring signal is squared in the clock signal estimation unit 71, and is then filtered using a bandpass filter having a narrow passband which is centered about the symbol clock frequency, and clock phase is determined from the phase of a clock frequency component which is contained in the filtered output. A peak in the line spectrum of the symbol frequency components does exist in the squared IF signal for the .pi./4 DQPSK or QPSK signal, but such peak does not exist in the OQPSK signal, and hence the described technique is inapplicable. In addition, for OQPSK signal, a data demodulation according to the delayed detection is prohibited due to the presence of a crosstalk between I and Q. On the other hand, a data demodulation according to the delayed detection is possible with .pi./4 DQPSK or QPSK signal. Additionally, it happens in the prior art that the processing operations mentioned under the subparagraph 1-3 must be repeated many times in some instance, resulting in an increased computational complexity and an increased length of time.
According to the prior art, data demodulation and estimation of transmission parameters are prohibited without a modulation scheme which satisfies the requirements: a) delayed detection is possible and b) an estimation of a clock phase is possible without using transmitted data. Taking an example of OQPSK modulation signal, for example, it does not satisfy these requirements, and accordingly, with a conventional measuring algorithm, a waveform quality measurement is prohibited.
It is an object of the invention to provide a measuring apparatus which enables a data demodulation (or data detection) and an estimation of transmission parameters for a modulation signal from a digital equipment which does not satisfy the requirements mentioned above.
For a modulation scheme which does not permit a delayed detection, a data demodulation must be achieved through a synchronous detection. In the synchronous detection, it is required that a carrier frequency and a carrier phase of a received signal (or measuring signal) be known. On the other hand, an estimation of a carrier phase as one of transmission parameters requires demodulated data. Thus, with prior art, an estimation of transmission parameters which accommodates for any modulation scheme has been difficult.